Monoclonal antibodies (MAbs) to outer surface protein B (Osp B) of Borrelia burgdorferi, the etiological agent of Lyme disease, have shown to be bactericidal for these spirochetes without the intervention of complement. These MAbs have also selected escape variants which do not express Osp B. The phenotype of the Osp B- variants indicate that there are spontaneous mutations in the reading frame of the Osp since the genes for both Osp A and Osp B are still present. These observations have led to the formulation of the hypotheses to be tested in this proposal. The first is that the killing mechanism of these antibodies is novel, The second is that an understanding of the killing mechanism(s) may also produce significant insights into the functional role of the Osp. Under Specific Aim I, we are proposing to investigate the avidity, affinity and bactericidal kinetics of these MAbs, and compare these observations with those obtained from MAbs which are not bactericidal. In addition, we will map their epitopes, identify the mutations of the selected variants, and examine the possible role of Osp B as a porin. The availability of variants lacking one or both of the Osp, affinity purified native Osp A and B lipoproteins, and recombinant Osp will allow definitive statements regarding the role of these antigens in adhesion to and cytotoxicity for eukaryotic cells. Adhesion of B. burgdorferi to oligodendroglia results in cell death. The morphology of the dying cells is characteristic of apoptosis where the cells undergo chromatin condensation around the inner periphery of the nuclear membrane, retraction of cytoplasm, and blebbing. Under Specific Aim II, we are proposing to explore the hypothesis that the surface interaction of spirochetes with neural cells of low mitotic activity results in a cellular response leading to death. We will examine whether Osp become incorporated onto the surface or are internalized by the neural cells, assay for ion mobilization, elevation of cAMP, and for the production of endonucleases leading to DNA fragmentation.